The rabbit corpus luteum responds to 17 beta-estradiol which is the essential luteotropic hormone in this species. We propose to investigate the mechanism of action of estradiol in the corpus luteum and to determine how estradiol and LH cooperatively regulate progesterone synthesis. Two types of experimental animals will be used, in which rapid steroidogenic responses to estrogen can be obtained: 1) the pseudopregnant rabbit treated continuously with estradiol by a Silastic implant which can be removed to initiate a rapid decline in luteal progesterone synthesis; 2) the pseudopregnant rabbit injected with hCG to cause a decrease in follicular estrogen synthesis and in luteal progesterone synthesis. In both animals, estradiol can be administered to restore progesterone synthesis. Corpora lutea will be removed at intervals close to the time of estradiol withdrawal or stimulation and the changes in cell constituents suspected to be linked to estrogen action and progesterone synthesis measured. These include estrogen receptor, certain key steroidogenic enzyme activities (cholesterol side-chain lyase, cholesteryl esterase, HMG CoA reductase, cytochrome P-450 delta 5, 3 beta-hydroxysteroid dehydrogenase), adenylate cyclase activities. Corpora lutea, in which progesterone synthesis has been stimulated by estradiol in vivo, will be incubated with inhibitors of RNA and protein synthesis to determine the effects of progesterone synthesis in vitro. The relationships between estrogen and LH actions will be investigated by determining the acute effects of estradiol stimulation and withdrawal on LH-stimulated progesterone synthesis, LH receptor, LH-sensitive adenylate cyclase activity, and selected steroidogenic enzymes responsive to LH. The effect of estradiol alone and in combination with LH on luteal progesterone synthesis will be investigated in hypophysectomized rabbits to determine if these two hormones act synergistically.